[18F]-FDG positron emission tomography--an established clinical tool opening a new window into exercise physiology

Alterations in Leg Muscle Glucose Uptake and Inter-Limb Asymmetry after a Single Session of tDCS in Four People with Multiple Sclerosis

Asymmetrical lower limb weakness is an early symptom and significant contributor to the progressive worsening of walking ability in people with multiple sclerosis (PwMS). Transcranial direct current stimulation (tDCS) may effectively increase neural drive to the more-affected lower limb and, therefore, increase symmetrical activation. Four PwMS (1 female, age range: 27–57) underwent one session each of 3 mA or SHAM tDCS over the motor cortex corresponding to their more-affected limb followed by 20 min of treadmill walking at a self-selected speed. Two min into the treadmill task, the subjects were injected with the glucose analog [¹⁸F]fluorodeoxyglucose (FDG). Immediately after treadmill walking, the subjects underwent whole-body positron emission tomography (PET) imaging. Glucose uptake (GU) values were compared between the legs, the spatial distribution of FDG was assessed to estimate glucose uptake heterogeneity (GUh), and GU asymmetry indices (AIs) were calculated. After tDCS, GU was altered, and GUh was decreased in various muscle groups in each subject. Additionally, AIs went from asymmetric to symmetric after tDCS in the subjects that demonstrated asymmetrical glucose uptake during SHAM. These results indicate that tDCS improved GU asymmetries, potentially from an increased neural drive and a more efficient muscle activation strategy of the lower limb in PwMS.

Exercise Physiology From 1980 to 2020: Application of the Natural Sciences

The field of exercise physiology has enjoyed tremendous growth in the past 40 years. With its foundations in the natural sciences, it is an interdisciplinary field that is highly relevant to human performance and health. The focus of this review is on highlighting new approaches, knowledge, and opportunities that have emerged in exercise physiology over the last four decades. Key among these is the adoption of advanced technologies by exercise physiologists to address fundamental research questions, and the expansion of research topics to range from molecular to organismal, and population scales in order to clarify the underlying mechanisms and impact of physiological responses to exercise in health and disease. Collectively, these advances have ensured the position of the field as a partner in generating new knowledge across many scientific and health disciplines.

Systemic cross-talk between brain, gut, and peripheral tissues in glucose homeostasis: effects of exercise training (CROSSYS). Exercise training intervention in monozygotic twins discordant for body weight

BACKGROUND

Obesity and physical inactivity are major global public health concerns, both of which increase the risk of insulin resistance and type 2 diabetes. Regulation of glucose homeostasis involves cross-talk between the central nervous system, peripheral tissues, and gut microbiota, and is affected by genetics. Systemic cross-talk between brain, gut, and peripheral tissues in glucose homeostasis: effects of exercise training (CROSSYS) aims to gain new systems-level understanding of the central metabolism in human body, and how exercise training affects this cross-talk.

METHODS

CROSSYS is an exercise training intervention, in which participants are monozygotic twins from pairs discordant for body mass index (BMI) and within a pair at least the other is overweight. Twins are recruited from three population-based longitudinal Finnish twin studies, including twins born in 1983-1987, 1975-1979, and 1945-1958. The participants undergo 6-month-long exercise intervention period, exercising four times a week (including endurance, strength, and high-intensity training). Before and after the exercise intervention, comprehensive measurements are performed in Turku PET Centre, Turku, Finland. The measurements include: two positron emission tomography studies (insulin-stimulated whole-body and tissue-specific glucose uptake and neuroinflammation), magnetic resonance imaging (brain morphology and function, quantification of body fat masses and organ volumes), magnetic resonance spectroscopy (quantification of fat within heart, pancreas, liver and tibialis anterior muscle), echocardiography, skeletal muscle and adipose tissue biopsies, a neuropsychological test battery as well as biosamples from blood, urine and stool. The participants also perform a maximal exercise capacity test and tests of muscular strength.

DISCUSSION

This study addresses the major public health problems related to modern lifestyle, obesity, and physical inactivity. An eminent strength of this project is the possibility to study monozygotic twin pairs that share the genome at the sequence level but are discordant for BMI that is a risk factor for metabolic impairments such as insulin resistance. Thus, this exercise training intervention elucidates the effects of obesity on metabolism and whether regular exercise training is able to reverse obesity-related impairments in metabolism in the absence of the confounding effects of genetic factors.

TRIAL REGISTRATION

ClinicalTrials.gov , NCT03730610 . Prospectively registered 5 November 2018.

Adsorption separation of heavier isotope gases in subnanometer carbon pores

Isotopes of heavier gases including carbon (¹³C/¹⁴C), nitrogen (¹³N), and oxygen (¹⁸O) are highly important because they can be substituted for naturally occurring atoms without significantly perturbing the biochemical properties of the radiolabelled parent molecules. These labelled molecules are employed in clinical radiopharmaceuticals, in studies of brain disease and as imaging probes for advanced medical imaging techniques such as positron-emission tomography (PET). Established distillation-based isotope gas separation methods have a separation factor (S) below 1.05 and incur very high operating costs due to high energy consumption and long processing times, highlighting the need for new separation technologies. Here, we show a rapid and highly selective adsorption-based separation of ¹⁸O₂ from ¹⁶O₂ with S above 60 using nanoporous adsorbents operating near the boiling point of methane (112 K), which is accessible through cryogenic liquefied-natural-gas technology. A collective-nuclear-quantum effect difference between the ordered ¹⁸O₂ and ¹⁶O₂ molecular assemblies confined in subnanometer pores can explain the observed equilibrium separation and is applicable to other isotopic gases.

Total-Body PET Imaging of Musculoskeletal Disorders

Imaging of musculoskeletal disorders, including arthritis, infection, osteoporosis, sarcopenia, and malignancies, is often limited when using conventional modalities such as radiography, computed tomography (CT), and MR imaging. As a result of recent advances in Positron Emission Tomography (PET) instrumentation, total-body PET/CT offers a longer axial field-of-view, higher geometric sensitivity, and higher spatial resolution compared with standard PET systems. This article discusses the potential applications of total-body PET/CT imaging in the assessment of musculoskeletal disorders.

Different Effects of Transcranial Direct Current Stimulation on Leg Muscle Glucose Uptake Asymmetry in Two Women with Multiple Sclerosis

Asymmetrical lower limb strength is a significant contributor to impaired walking abilities in people with multiple sclerosis (PwMS). Transcranial direct current stimulation (tDCS) may be an effective technique to enhance cortical excitability and increase neural drive to more-affected lower limbs. A sham-controlled, randomized, cross-over design was employed. Two women with MS underwent two 20 min sessions of either 3 mA tDCS or Sham before 20 min of treadmill walking at a self-selected speed. During walking, the participants were injected with the glucose analogue, [¹⁸F] fluorodeoxyglucose (FDG). Participants were then imaged to examine glucose metabolism and uptake asymmetries in the legs. Standardized uptake values (SUVs) were compared between the legs and asymmetry indices were calculated. Subject 2 was considered physically active (self-reported participating in at least 30 min of moderate-intensity physical activity on at least three days of the week for the last three months), while Subject 1 was physically inactive. In Subject 1, there was a decrease in SUVs at the left knee flexors, left upper leg, left and right plantar flexors, and left and right lower legs and SUVs in the knee extensors and dorsiflexors were considered symmetric after tDCS compared to Sham. Subject 2 showed an increase in SUVs at the left and right upper legs, right plantar flexors, and right lower leg with no muscle group changing asymmetry status. This study demonstrates that tDCS may increase neural drive to leg muscles and decrease glucose uptake during walking in PwMS with low physical activity levels.

Imaging Transcranial Direct Current Stimulation (tDCS) with Positron Emission Tomography (PET)

Transcranial direct current stimulation (tDCS) is a form of non-invasive neuromodulation that is increasingly being utilized to examine and modify several cognitive and motor functions. Although tDCS holds great potential, it is difficult to determine optimal treatment procedures to accommodate configurations, the complex shapes, and dramatic conductivity differences among various tissues. Furthermore, recent demonstrations showed that up to 75% of the tDCS current applied to rodents and human cadavers was shunted by the scalp, subcutaneous tissue, and muscle, bringing the effects of tDCS on the cortex into question. Consequently, it is essential to combine tDCS with human neuroimaging to complement animal and cadaver studies and clarify if and how tDCS can affect neural function. One viable approach is positron emission tomography (PET) imaging. PET has unique potential for examining the effects of tDCS within the central nervous system in vivo, including cerebral metabolism, neuroreceptor occupancy, and neurotransmitter activity/binding. The focus of this review is the emerging role of PET and potential PET radiotracers for studying tDCS-induced functional changes in the human brain.

Fluorine-18-fluoro-2-deoxy-d-glucose PET-CT aids in detection of soft-tissue injuries for dogs with thoracic or pelvic limb lameness

Fluorine-18-fluoro-2-deoxy-d-glucose positron emission tomography (FDG PET) provides physiologic images of tissues based on their glucose metabolism. The combination of FDG PET and CT (FDG PET-CT) has been utilized in human musculoskeletal imaging to localize soft tissue lesions, however, this modality has not been thoroughly investigated for the diagnosis of canine lameness. This prospective, descriptive study evaluated FDG PET-CT findings in 25 client-owned dogs with inconclusive origin of thoracic or pelvic limb lameness (thoracic limb n = 15/25, 60%; pelvic limb n = 6/25, 24%; and combination of both limbs n = 4/25, 16%). We hypothesized that FDG PET-CT would aid the detection of soft tissue lesions not visible with other imaging modalities. Combined FDG PET-CT detected soft tissue lesions in 40% (n = 10/25) and osteoarthritis in 64% (n = 16/25) of the patients. FDG PET detected more soft tissue lesions than contrast-enhanced CT (n = 15/15, 100% and n = 12/15, 80%, respectively), while CT identified more osteoarthritis lesions than FDG PET (n = 26/26, 100% and n = 18/26, 69%, respectively). The three imaging-diagnoses based on the FDG PET component included the following: flexor carpi ulnaris muscle tear, psoas major myopathy, and tarsal desmopathy. No diagnosis for the lameness was obtained in three dogs. Findings supported FDG PET-CT as a useful adjunct imaging modality for detection of certain soft tissue injuries of the musculoskeletal system. Combined FDG PET-CT should be considered for cases where the cause of lameness is thought to be of soft tissue origin and cannot be diagnosed by conventional means.

Effects of Muscle Function and Limb Loading Asymmetries on Gait and Balance in People With Multiple Sclerosis

People with MS (PwMS) often have a more- and less-affected side of the body which results in a variety of asymmetries, including measures of power, strength, muscle activity, and limb loading. Though many studies have identified asymmetries, their impact on gait and balance in PwMS is currently unclear. In this mini-review we first summarize previous findings of asymmetries in muscle function and limb loading and their impact on gait and balance in PwMS. We then provide potential explanations for this lack of consistency in the current literature, and propose study guidelines to improve future lower limb asymmetry studies. Making use of a unified approach to study lower limb asymmetry may then provide more clarity regarding their impact on mobility, specifically gait and balance, in PwMS.

Shivering and nonshivering thermogenesis in skeletal muscles

Humans have inherited complex neural circuits which drive behavioral, somatic, and autonomic thermoregulatory responses to defend their body temperature. While they are well adapted to dissipate heat in warm climates, they have a reduced capacity to preserve it in cold environments. Consequently, heat production is critical to defending their core temperature. As in other large mammals, skeletal muscles are the primary source of heat production recruited in cold-exposed humans. This is achieved voluntarily in the form of contractions from exercising muscles or involuntarily in the form of contractions from shivering muscles and the recruitment of nonshivering mechanisms. This review describes our current understanding of shivering and nonshivering thermogenesis in skeletal muscles, from the neural circuitry driving their recruitment to the metabolic substrates that fuel them. The presence of these heat-producing mechanisms can be measured in vivo by combining indirect respiratory calorimetry with electromyography or biomedical imaging modalities. Indeed, much of what is known regarding shivering in humans and other animal models stems from studies performed using these methods combined with in situ and in vivo neurologic techniques. More recent investigations have focused on understanding the metabolic processes that produce the heat from both contracting and noncontracting mechanisms. With the growing interest in the potential therapeutic benefits of shivering and nonshivering skeletal muscle to counter the effects of neuromuscular, cardiovascular, and metabolic diseases, we expect this field to continue its growth in the coming years.

Metabolic imaging in exercise physiology

This minireview focuses on selected, noninvasive imaging techniques that have been used in the study of exercise physiology. These imaging modalities can be roughly divided into two categories: tracer based and nontracer based. Tracer-based methods use radiolabeled substrates whose location and quantity can subsequently be imaged once they are incorporated into metabolic processes. Nontracer-based imaging modalities rely on specific properties of substrates to identify metabolites and determine their concentrations. Identification and quantification of metabolites is usually based on magnetic properties or on differences in light absorption. In this review, we will highlight two tracer-based imaging modalities, positron emission tomography and single-photon-emission computed tomography, as well as two nontracer-based methods, magnetic resonance spectroscopy and near-infrared spectroscopy. Some of the recent findings that each technique has provided on cerebral and skeletal muscle metabolism during exercise, as well as the strengths and limitations of each technique, will be discussed.

Assessment of muscle function using hybrid PET/MRI: comparison of 18F-FDG PET and T2-weighted MRI for quantifying muscle activation in human subjects

Purpose

The aim of this study was to determine the relationship between relative glucose uptake and MRI T₂ changes in skeletal muscles following resistance exercise using simultaneous PET/MRI scans.

Methods

Ten young healthy recreationally active men (age 21 – 28 years) were injected with ¹⁸F-FDG while activating the quadriceps of one leg with repeated knee extension exercises followed by hand-grip exercises for one arm. Immediately following the exercises, the subjects were scanned simultaneously with ¹⁸F-FDG PET/MRI and muscle groups were evaluated for increases in ¹⁸F-FDG uptake and MRI T₂ values.

Results

A significant linear correlation between ¹⁸F-FDG uptake and changes in muscle T₂ (R² = 0.71) was found. for both small and large muscles and in voxel to voxel comparisons. Despite large intersubject differences in muscle recruitment, the linear correlation between ¹⁸F-FDG uptake and changes in muscle T₂ did not vary among subjects.

Conclusion

This is the first assessment of skeletal muscle activation using hybrid PET/MRI and the first study to demonstrate a high correlation between ¹⁸F-FDG uptake and changes in muscle T₂ with physical exercise. Accordingly, it seems that changes in muscle T₂ may be used as a surrogate marker for glucose uptake and lead to an improved insight into the metabolic changes that occur with muscle activation. Such knowledge may lead to improved treatment strategies in patients with neuromuscular pathologies such as stroke, spinal cord injuries and muscular dystrophies.

18F-FDG Positron Emission Tomography - An Innovative Technique for the Diagnosis of a Canine Lameness

Introduction

Positron emission tomography (PET) imaging with fluorine-18-fluorodeoxyglucose (18F-FDG) is widely known for its use in the diagnosis and tracking of primary and metastatic tumors via uptake and retention of the radiopharmaceutical by hypermetabolic cells. 18F-FDG is also used to study the normal physiology of glucose uptake, metabolism, and muscle activity during and after exercise.

Background

A pilot study adding PET imaging to the diagnostic evaluation of canine patients undergoing computed tomography (CT) for mild or intermittent thoracic and pelvic limb lameness is ongoing. Dogs with an observable (grade 1–2/5) lameness that have undergone routine radiography and complete physical examination by board-certified veterinary surgeons and sports medicine and rehabilitation specialists are enrolled. Each patient undergoes leash walking for 15 min prior to premedication and induction of general anesthesia for the PET–CT examination. 18F-FDG is injected intravenously, and a whole-body PET examination is conducted after 1 h of radiopharmaceutical uptake time. Standard algorithm, whole-body pre- and post-contrast CT examinations, and focused, standard, and bone algorithm CT scans of the thoracic or pelvic limb areas of interest are obtained concurrently. Abnormal PET–CT findings are further investigated with additional diagnostic imaging or at surgery (e.g., ultrasound, MRI, and arthroscopy).

Discussion

This case report uses a canine patient referred for thoracic limb lameness to illustrate the role of advanced imaging in a diagnostic plan and to discuss a recommended PET–CT procedure for lameness evaluation. The PET–CT imaging protocol recommended in this report was designed to significantly enhance a routine thoracic limb CT examination and to identify areas of muscle, tendon, or ligament overuse, inflammation, or injury for further diagnostic procedures or definitive treatment.

Concluding remarks

18F-FDG PET–CT adds valuable physiologic and anatomic information to the diagnostic evaluation of patients presenting with indistinct or intermittent clinical signs of musculoskeletal inflammation or injury. In addition, tailoring the PET acquisition and radiopharmaceutical parameters allows for detailed information gathering to more closely assess normal and abnormal physiology, unlocking a new frontier in the study of canine athletic injury and optimal performance.

FDG-PET detects nonuniform muscle activity in the lower body during human gait

INTRODUCTION

Nonuniform muscle activity has been partially explained by anatomically defined neuromuscular compartments. The purpose of this study was to investigate the uniformity of skeletal muscle activity during walking.

METHODS

Eight participants walked at a self-selected speed, and muscle activity was quantified using [¹⁸ F]-fluorodeoxyglucose positron emission tomography imaging. Seventeen muscles were divided into 10 equal length sections, and within muscle activity was compared.

RESULTS

Nonuniform activity was detected in 12 of 17 muscles (ƒ > 4.074; P < 0.046), which included both uni- and multi-articular muscles. Greater proximal activity was detected in 6 muscles (P < 0.049), and greater distal versus medial activity was found in the iliopsoas (P < 0.042).

CONCLUSIONS

Nonuniform muscle activity is likely related to recruitment of motor units located within separate neuromuscular compartments. These findings indicate that neuromuscular compartments are recruited selectively to allow for efficient energy transfer, and these patterns may be task-dependent. Muscle Nerve 54: 959-966, 2016.

Different Predictors of Right and Left Ventricular Metabolism in Healthy Middle-Aged Men

Dysfunction of the right ventricle (RV) plays a crucial role in the outcome of various cardiovascular diseases. Previous studies on RV metabolism are sparse although evidence implies it may differ from left ventricular (LV) metabolism. Therefore, the aims of this study were (1) to determine predictors of RV glucose uptake (GU) and free fatty acid uptake (FFAU) and (2) to compare them to predictors of LV metabolism in healthy middle-aged men. Altogether 28 healthy, sedentary, middle-aged (40-55 years) men were studied. Insulin-stimulated GU and fasting FFAU were measured by positron emission tomography and RV and LV structural and functional parameters by cardiac magnetic resonance. Several parameters related to whole-body health were also measured. Predictors of RV and LV metabolism were determined by pairwise correlation analysis, lasso regression models, and variable clustering using heatmap. RVGU was most strongly predicted by age and moderately by RV ejection fraction (EF). The strongest determinants of RVFFAU were exercise capacity (peak oxygen uptake), resting heart rate, LVEF, and whole-body insulin-stimulated glucose uptake rate. When considering LV metabolism, age and RVEF were associated also with LVGU. In addition, LVGU was strongly, and negatively, influenced by whole-body insulin-stimulated glucose uptake rate. LVFFAU was predicted only by LVEF. This study shows that while RV and LV metabolism have shared characteristics, they also have unique properties. Age of the subject should be taken into account when measuring myocardial glucose utilization. Ejection fraction is related to myocardial metabolism, and even so that RVEF may be more closely related to GU of both ventricles and LVEF to FFAU of both ventricles, a finding supporting the ventricular interdependence. However, only RV fatty acid utilization associates with exercise capacity so that better physical fitness in a relatively sedentary population is related with decreased RV fat metabolism. To conclude, this study highlights the need for further study designed specifically on less-known RV, as the results on LV metabolism and physiology may not be directly applicable to the RV.